1. Field of the Invention
The present invention relates to a wirecut electric discharge machine (hereinafter referred to as a "wirecut EDM") and particularly to an apparatus for threading a wire electrode through a wire guide.
2. Description of the Background Art
FIG. 6 shows a conventional wire electrode supplying apparatus for a wirecut EDM, of the type disclosed in Japanese Patent Disclosure Bulletin No. 1981-10130 or U.S. Pat. No. 4,495,393. It illustrates an apparatus for transferring a wire electrode 3, as confined by a jet fluid stream, e.g. jet water stream 7, from a wire electrode supplying wire guide body 4 to a wire electrode receiving wire guide body 5, the two bodies being disposed across a workpiece 1. Referring to FIG. 6, the workpiece 1 has an EDMing starting hole 2 drilled therein. Prior to the initiation of an EDM operation on the workpiece 1, the wire electrode 3 must be threaded from the wire electrode supplying wire guide body 4, through the hole 2, to the wire electrode receiving wire guide body 5. To this end, a jet nozzle 6 is formed within the wire guide body 4 and provides a jet fluid stream 7 along a line connecting a die guide 18 in guide body 4 and a die guide 9 in guide body 5. Similarly aligned are dielectric nozzles 10 and 11, located in wire guide bodies and 5, respectively. The dielectric is supplied from a dielectric supplying unit (not illustrated).
In such a structure the wire electrode 3 may be automatically threaded. To this end, the jet fluid stream is injected from the jet nozzle 6 provided in the wire electrode supplying wire guide body 4, and the wire electrode 3 is moved, confined by the jet fluid stream 7, by a wire electrode supplying mechanism (not illustrated) through the dielectric nozzle 10, the EDMing starting hole and the wire electrode receiving wire guide body 5, in this order. Then, it is wound on a wire collecting drum (not shown) or directed to a predetermined container. Wirecut EDMing is enabled upon the completion of such threading.
However, if the clearance between the lower dielectric nozzle 11 and the workpiece 1 is small and the diameter of the EDMing starting hole 2 is smaller than the outside diameter of the dielectric nozzle 11, as shown in FIG. 7, the jet fluid stream 7 strikes against and rebounds from the die guide 9. The rebounding fluid will fill the dielectric nozzle 11 as well as the EDMing starting hole 2. Since the jet fluid stream 7 is abruptly reduced in flow rate due to interference caused by the fluid in the filled nozzle and hole, and also encounters turbulence, it loses the power to confine the wire electrode 3. In addition, the wire electrode 3 is always curled to some degree because it ordinarily is wound around a supply bobbin for storage, and initially receives curl when produced in the wire drawing process. Further, since a pulley, a roller and the like are generally used in the path of the wire electrode 3 from the wire supply bobbin to the wire electrode supplying wire guide body in the wirecut EDM, a new curl may be added to the leading end of wire electrode 3 while it is running along the wire electrode path. Since the wire electrode 3 is curled due to composite factors as described above, the curling ratio, the curling direction, etc. of the wire electrode 3 will vary when the wire electrode 3 is uncoiled from the wire supply bobbin and is directed along the wire electrode path. Hence, when the jet fluid stream loses its ability to confine the wire electrode 3, it becomes difficult for the tip of the wire electrode 3 to be accurately fed into the die guide 9. As a result, the wire electrode 3 may be diverted away from the die guide 9 as illustrated in FIG. 7.
If a workpiece is not present between the dielectric nozzles 10 and 11, as shown in FIG. 8, the jet fluid stream 7 loses its power to confine the wire electrode 3 and the electrode may be diverted away from the die guide 9 or the dielectric nozzle 11, as described previously. As a result, the wire electrode supplying operation cannot be continued because the jet fluid stream 7 fills the dielectric nozzle 11 and the fluid rebounding from the dielectric nozzle 11 will interfere with the jet fluid stream 7.
One conventional solution to this problem is the use of a drain 22 in the dielectric nozzle 11, as seen in FIG. 9 and taught in Japanese Patent Bulletin 63-50131. During the threading operation, the fluid jet stream will guide the tip of wire electrode 3 through the hole 2 in the workpiece 1 and into the dielectric nozzle 11. The jet stream fluid is drained from the nozzle 11 by drain 22, thereby preventing the nozzle from being filled. However, the jet stream still causes turbulence when it encounters the die guide 9, which interferes with the accurate guiding of the tip of wire 3.